The Chiapas Massif, located in the southwestern Maya terrane, is primarily composed of igneous and metaigneous rocks of Late Permian - early Triassic age, although two distinct metasedimentary units have also been recognized. The Sepultura Unit, located in the nothwestern Chiapas Massif, is composed of psammites, pelites and calcsilicate rocks metamorphosed under high-T, low- to medium-P conditions. The Custepec Unit, which is the object of this study, is located in the central-southeastern portion of the Chiapas Massif and is composed of partially melted amphibolites interbedded with quartzofeldspathic and pelitic gneisses, marbles, calcsilicate rocks, and quartzites. Geothermobarometry of a metapelite and an amphibolite yielded consistent metamorphic conditions of ~800 °C and 9 kbar. These pressure and temperature estimates, along with petrographic observations indicate that metamorphic conditions were in the amphibolite-granulite facies transition at ~25-30 km depth. A Sm-Nd garnet-whole rock age of 268 ± 9 Ma, which predates a previously published U-Pb zircon age of anatectic and metamorphic zircons of 252 ± 4 Ma, indicates that Sm and Nd in garnet behaved as a closed system during high-grade metamorphism, while zircon crystallized at temperatures below the metamorphic peak. The high-grade metamorphic event was followed by a phase of isobaric cooling, medium grade retrogression reactions, uplift and greenschist facies diaphtoresis accompanied by further ductile deformation.